Donald J. Trump fired his divisive campaign manager, Corey Lewandowski, on Monday as he confronts urgent challenges heading into the general election — a strategic shift after months of concerns from party officials and donors about Mr. Lewandowski’s stewardship of the campaign.

The exit occurred a month before Mr. Trump, the presumptive Republican presidential nominee, is to secure the official nomination at the party’s convention in Cleveland. It reflected a broader adjustment by the campaign as it grapples with a late start to fund-raising, anxiety among party leaders and a skeletal staff — all while Mr. Trump’s likely Democratic opponent, Hillary Clinton, swiftly builds out her operations in swing states.

Mr. Trump had faced increasing concerns from allies and donors, as well as his children, over whether Mr. Lewandowski, who had never before worked on a national race, was able to direct a battle against Mrs. Clinton. Among those who had voiced concern was Reince Priebus, the chairman of the Republican National Committee, who told Mr. Trump last week that relations between his committee and Mr. Lewandowski had become increasingly strained, and that a change would be welcome, according to three people briefed on the discussion.

Republicans across the spectrum welcomed the firing as a positive step, but they suggested that it needed to be followed by consistent changes in performance from the candidate himself.

Mr. Lewandowski was fired at a Monday morning meeting with Mr. Trump and Mr. Trump’s two older sons, Eric and Donald Jr., said two others briefed on the meeting,who were not authorized to speak publicly. Mr. Trump and Mr. Lewandowski had what was described as a “very open conversation.”

Mr. Lewandowski’s time was primarily spent on the campaign trail with the candidate, and day-to-day aspects of the operation were largely handled by the chief strategist, Paul Manafort. For months, Mr. Lewandowski had been a lightning rod for controversy, making headlines about himself that overshadowed his boss. This included his being charged with misdemeanor battery — a charge later dropped — after he was accused of grabbing a reporter as she approached Mr. Trump in Florida in March.

It was not immediately clear whether someone new would be named campaign manager, but it was clear that the existing Trump team believed most of the duties had already been assumed by other people, principally Mr. Manafort.

“Ultimately, Paul is in charge,” Barry Bennett, a senior adviser to the Trump campaign, said. “He’s got the experience to help get Mr. Trump across the finish line.”

As a young Republican operative, Mr. Manafort helped manage the 1976 convention floor for Gerald Ford in his showdown with Ronald Reagan, the last time Republicans entered a convention with no candidate’s having clinched the nomination. He performed a similar function for Reagan in 1980, and played leading roles in the 1988 and 1996 conventions, for George Bush and Bob Dole.

Trump allies and critics alike regarded Mr. Lewandowski as a fierce defender of Mr. Trump’s idiosyncratic approach to the presidential race. At a moment when many in the party have pressed Mr. Trump to soften his message and build a more conventional political operation, Mr. Lewandowski hewed closely to the mantra he had developed during the Republican primaries: “Let Trump be Trump.”

The limitations of that approach have been on vivid display in recent weeks. Mr. Trump has struggled to raise money from establishment donors, and he has drawn fresh criticism from Republicans and Democrats for his racial attacks on a federal judge and his revived proposal to bar Muslims from entering the United States in the wake of the gay nightclub massacre in Orlando, Fla.

With the Republican National Convention looming, he faces the task of broadening his team to include people with previous presidential campaign experience and uniting a party that is often not in lock step behind him.

Mr. Trump has also been turning his attention to fund-raising for the first time, a task over which Mr. Lewandowski had assumed oversight, and one that has gone slowly for the campaign. The campaign has aired no ads for the general election, and neither Mr. Trump or his advisers have yet to publicly bless a “super PAC” that could raise significant amounts of money to support his presidential bid.

“It would be welcome if new people come in who have more experience and can move him to a more inclusive, more substance-oriented campaign,” said Fred Malek, a fixture in Republican Party fund-raising.

But he added: “How much the absence of a national kind of campaign is due to Corey and how much is due to Donald is kind of hard to tell. It looks to me like Trump drives his own train.”

In announcing Monday morning that Mr. Lewandowski “will no longer be working with the campaign,” Hope Hicks, the campaign’s spokeswoman, said in a statement that “the campaign is grateful to Corey for his hard work and dedication, and we wish him the best in the future.”

The firing followed regular reports of turmoil in the campaign. Mr. Lewandowski was often at odds with Mr. Manafort, who was brought on in March when the candidate seemed poised for a lengthy fight over Republican delegates.

Mr. Lewandowski was said to have resisted certain moves that would have increased the number of staff members, at times blocking Mr. Manafort from making hires or later undoing them.

But the people briefed on Mr. Lewandowski’s departure said the circumstances went well beyond any one episode or relationship. One stressed that the move had been in the works for many weeks, particularly since it had become clear that Mr. Trump would be the Republican nominee.

Mr. Trump’s son Donald Jr. described the split as “amicable” in an interview with NBC. And Mr. Lewandowski gave a series of interviews Monday afternoon in which he tried to brush aside questions about the internal particulars of his departure. He said he wished nothing but the best for Mr. Trump, and suggested that Mr. Trump’s doing well could only be good for him.

“If Donald Trump wins, that’s good for Corey Lewandowski,” Mr. Lewandowski said on CNN. He played down any suggestion of tension between him and Mr. Trump’s children, and said every campaign expanded its operations for a general election.

Few inside the campaign were given any warning about the dismissal of Mr. Lewandowski, who was on the campaign’s daily 8:30 a.m. conference call on Monday, according to a person briefed on the developments.

Mr. Bennett, the senior Trump campaign adviser, declined to predict whether there would be other significant changes. But he said Mr. Lewandowski deserved credit for helping Mr. Trump get where he is.

“There is no doubt what Corey did in the primary was amazing — helping him get more votes than anyone else has ever gotten, a record turnout in 45 out of the 50 states, record low expenditures,” Mr. Bennett said. “None of that’s ever been done before.”

A version of this article appears in print on , on Page A15 of the New York edition with the headline: With the Pressure Rising, Trump Removes His Campaign Manager. Order Reprints | Today’s Paper | Subscribe